1. Field of the Invention
The present invention relates to a key controlled digital system and is adapted mainly for electronic desk-top computers or calculators.
2. Description of the Prior Art
In a key controlled digital system such as an electronic desk-top computer or calculator, the main portion of the digital circuit is constituted in a form of MIS (Metal-Insulator-Semiconductor) integrated circuit. In general, the number of the external terminals of semiconductor integrated circuits is severely limited. In the case of the electronic desk-top computer, therefore, signals from a keyboard are first converted to timing pulses and then applied to the external terminals of the semiconductor integrated circuit in a multiplex fashion so that the number of the external terminals may be reduced.
An example of such a digital system as described above is disclosed in U.S. Pat. No. 3,781,852. According to this patent, a multidigit numerical display device is energized by a repeating sequential series of digit pulses and a group of segment pulses. The digit pulses are coupled to the keys of a keyboard and are input to a digital computing module when the associated keys are depressed or actuated. Coding for the actuated key is performed internally of the computing module. In this digital system, however, the digit pulses and the segment pulses are both applied to the display device even during a key input period in which the digit pulses are coupled to the keyboard and hence the computing module, so that the electric power consumption by the display device is comparatively large. Moreover, in the case where fluorescent display tubes are used for the display device, the electric power consumption by the tubes becomes relatively large so that the current capacity of the digit pulse supplying drive circuit constituted of MISFET's must be large. Accordingly, the loss in the output level of the digit pulse on the side of a reference potential becomes large. This results in the deterioration of key input level.